1. Technical Field
The present invention relates to a quantitative analyzing method of a copper indium gallium selenide (CIGS) film using a laser induced breakdown spectroscopy.
2. Description of the Related Art
Plasma generated by laser irradiation emits light having a specific wavelength depending on the material on which the laser is irradiated. As a result, components of the material may be qualitatively or quantitatively analyzed by collecting the light. A laser induced breakdown spectroscopy (hereinafter, referred to as LIBS), which is one method of analyzing the components of the material using the collected light, is a spectroscopic analysis technology using plasma produced by generating breakdown, which is a kind of discharge phenomenon, using a high output laser, as an excitation source. A sample is vaporized in the plasma induced by the laser, such that atoms and ions may be present in an excited state. The atoms and ions in the excited state release energy after a lifespan and return back to a ground state. In this case, the atoms and ions emit light having a unique wavelength according to the kind of elements and the excited state. Therefore, when analyzing a spectrum of the emitted light, the components of the material may be qualitatively or quantitatively analyzed.
FIG. 1 is an illustration view showing an operation principle of LIBS according to the related art.
Referring to FIG. 1, first, in the case in which an ablation (a phenomenon in which the material is removed while being melted and evaporated by the laser) is performed for a material having a very small quantity (several μg) by irradiating a pulse laser, as in Step 102, the ablated material absorbs laser energy to thereby cause ionization in a very short time (typically, in several nanoseconds), and to form high temperature plasma of about 15000 K or more as in Step 104. When a laser pulse is stopped, the respective elements present in the plasma emit specific spectra corresponding thereto while the high temperature plasma is cooled. In this case, by collecting and analyzing the emitted spectra using a spectrometer as in Step 106, unique spectrum data of each element may be obtained as in Step 108 and component composition and quantity of substance contained in the material may be measured by analyzing the spectrum data.
The LIBS technology is different from other measuring technologies in that 1) an entire time spent on measuring is within 1 second, 2) a separate sampling and pre-conditioning process for the measurement is not required, 3) since only a very small quantity (several μg) of material is consumed for one measurement, an elementary composition of the material may be measured precisely to nm unit while the material is ablated in a depth direction, 4) a separate environment for the measurement is not required and the measurement may be performed under air atmosphere, 5) all elements except for an inert gas may be analyzed in ppm precision, and 6) an instrument may be configured at relatively low costs.
FIG. 2 is a chart comparing the LIBS with other measuring technologies.
Referring to FIG. 2, since a secondary ion mass spectrometry (SIMS), an atomic emission spectroscopy (AES), an energy dispersive X-ray spectroscopy (EDS), a glow discharge mass spectrometry (GD-MS), and the like which are frequently used in measuring a substance distribution need to be performed under high vacuum, it is only possible to measure in a laboratory specific to the invention thereof and it is impossible to practically apply to a production line. Since an inductively coupled plasma mass spectrometry (ICP-MS) which is widely used other than those mentioned above has difficulty in that a piece to be analyzed needs to be melted in a solvent and should then be analyzed, it is also impossible to apply to the production line. Currently, an X-ray fluorescence (XRF), which is widely used for analyzing substance of a solar cell material in the laboratory or in the field due to simplicity of use is relatively inexpensive and may be measured under air atmosphere, but has a technical limitation in measuring the substance distribution of a copper indium gallium selenide (CIGS) film in that {circle around (1)} since light elements such as Na, O, N, C, B, Be, Li, and the like are hardly measured, it is impossible to measure a Na content in the CIGS film, which has a decisive effect on a component efficiency, {circle around (2)} the XRF has a precision in a depth direction of at most about 1 μm, it is impossible to measure the element distribution in the depth direction in the CIGS film having a thickness of 2 μm, and {circle around (3)} it is difficult to determine whether a fluorescence signal to be measured is output from a practical film or a substrate.
In general, a semiconductor solar cell refers to a device of directly converting solar light into electricity using a photovoltaic effect in which electrons are generated when irradiating light on a semiconductor diode comprised of a p-n junction. As most basic configuration components, there are three portions such as a front electrode, a back contact electrode, and a light absorbing layer disposed therebetween. Among these, the most important material is the light absorbing layer that determines most of photoelectric transformation efficiency, and the solar cell is classified into various kinds according to the above-mentioned material. Particularly, a CIGS film solar cell refers to that in which the material of the light absorbing layer is made of Cu(In, Ga) Se2 which is a I-II-VI2 compound. The CIGS film solar cell, which is a high efficiency and low cost type solar cell, has recently been competitively marketed globally, has been prominent as the surest second-generation solar cell replacing a crystalline silicon solar cell in a solar cell field, and represents efficiency closest to a single crystalline silicon component, which is the maximal efficiency of 20.6%.
FIG. 3 is an illustration view schematically showing a structure of the CIGS film solar cell.
FIG. 4 is a flow chart schematically showing a process of manufacturing a CIGS film module.
Firstly, the CIGS film solar cell is manufactured by sequentially depositing a Mo layer, a CIGS layer, a CdS layer, and a TCO layer on a substrate. A detailed description thereof is as follows. The CIGS film module is manufactured by firstly depositing Mo, which is a back contact electrode layer on the substrate, forming (P1 scribing) a pattern by a scribing process, sequentially depositing the CIGS layer (the absorbing layer) and a CdS buffer layer on the Mo layer having the pattern formed thereon, forming (P2 scribing) a pattern by the scribing process, then sequentially depositing a transparent conductive oxide (TCO) layer and a front electrode grid made of Ni/Al on the CdS layer, and finally forming (P3 scribing) a pattern by performing the scribing process. The scribing process as described above is a process performing the patterning so as to be connected in series at a constant interval in order to prevent a decrease in efficiency due to an increase in a sheet resistance while an area of the solar cell is increased, and is performed over a total of three times, that is, P1, P2, and P3. According to the related art, the P1 scribing process performs the patterning using a laser, and the P2 and P3 scribing processes perform the patterning using a mechanical method, but a technology in which all of the P1, P2, and P3 scribing processes perform the patterning using the laser has been recently developed.
In a case of the CIGS film solar cell as described above, it has been reported that a thickness (1 to 2.2 μm) of the film, a structure of the device, a composition of substance configuring the CIGS film which is a multinary compound, and an element distribution in the film have a decisive effect on light absorption and photoelectric transformation efficiency, that sodium (Na) diffused into a CIGS light absorbing layer from soda-lime glass which is widely used as the substrate during the process increases a charge concentration of the film (Nakada et al., Jpn. J. Appl. Phys., 36, 732 (1997)) or increases a CIGS single grain size to thereby decrease structural characteristic variation according to a composition change and improve photoelectric transformation efficiency (Rockett et al., Thin Solid Films 361-362 (2000), 330; Probst et al., Proc of the First World Conf. on Photovoltaic Energy, Conversion (IEEE, New York, 1994), p 144). The reports as mentioned above show that chemical properties of the light absorbing layer need to be controlled by measuring the substance distribution in the film in order to manage quality in the production line of the CIGS film solar cell.
Meanwhile, a continuous production process of the CIGS film solar cell is mainly classified into a roll-to-plate (hereinafter, referred to as R2P) process using a hard material substrate such as the soda-lime glass and a roll-to-roll (hereinafter, referred to as R2R) process using a soft material substrate such as a metal thin plate such as stainless steel, Ti, Mo, or Cu, a polymer film such as polyimide, or the like. At a current time in which the present application is filed, a line of the continuous production process is not provided with a system capable of measuring physical and chemical properties of the CIGS film having the decisive effect on performance of the product in real time, such that physical and chemical properties as mentioned above cannot but depend on values which are pre-determined in a research and development phase. In addition, even though the physical and chemical properties are deviated from a physical and chemical standard targeted by a practical production process, it is impossible to separately check, and the deviated physical and chemical properties cannot but be found through degradation in performance and quality in a phase of evaluating the final completed product, thereby causing significant loss of the product. The continuous production process as described above requires considerable effort and time in order to detect a physical and chemical variable causing the degradation in performance and quality of the product, thereby causing an increase in price and degradation in competitiveness. Therefore, a development of a process control system capable of measuring physical and chemical properties of the CIGS film formed in real time without the pre-conditioning process in the continuous production process line has been urgently demanded.
Meanwhile, in the case of measuring properties of the CIGS film by LIBS, a light generated from an atom of plasma induced by a laser is absorbed by other circumjacent atoms, such that an intensity of the light may be decreased. In the case of generating a self-absorption phenomenon, an intensity of a spectrum of a target element to be measured is non-linearly changed according to concentration thereof. As a result, a degree of precision of the measured value becomes decreased.